1. Field of the Invention
The present invention relates to an apparatus and a process for image encoding, and a computer-readable non-transitory medium storing an image encoding program for making a computer execute the process for image encoding.
2. Description of the Related Art
Currently, development of the digital recording technology (such as MPEG) for recording video images taken by crime prevention cameras and the like is proceeding. Although video recording can be realized by storing video data in recording media such as HDDs (hard disk drives), opto-magnetic disks, and the like, the volume occupied by the video data in the recording media varies with the compression ratio of the video data even when the recording time is unchanged. For example, in the crime prevention camera systems, the time for which video data is recorded is long, so that the amount of data to be stored becomes massive. Therefore, it is necessary to increase the compression ratio while maintaining the image quality.
U.S. Patent Application Publication No. 20090202169 discloses an image transmission system in which an image is divided into a region of interest and regions other than the region of interest. The images of the regions other than the region of interest is filtered through a low-pass filter so that the resolution of the images of the regions other than the region of interest is lowered. On the other hand, the image of the region of interest is not filtered through the low-pass filter. That is, the amount of data is reduced by lowering the resolution of the images of the regions other than the necessary region.
In addition, Japanese Unexamined Patent Publication No. 2004-193968 discloses a color image communication system in which reference image data is produced by decoding received image data, and comparison image data is produced by encoding the reference image data with a predetermined encoding parameter and decoding the encoded image data. Then, the reference image data and the comparison image data are compared in order to obtain an evaluation result. When the evaluation result is at a level higher than a certain level, the predetermined encoding parameter is changed so as to increase the compression ratio. When the evaluation result is at a level lower than the certain level, the predetermined encoding parameter is changed so as to decrease the compression ratio.
However, according to the technique disclosed in U.S. Pat. No. 7,801,337, the filtering with the low-pass filter is not applied to the region of interest. Therefore, there is a possibility that the amount of video data cannot be sufficiently reduced in the case where the resolution of the image of the region of interest is allowed to be further reduced.
In addition, according to the technique disclosed in Japanese Unexamined Patent Publication No. 2004-193968, the encoding parameter and the like are changed after the encoding of the image data is completed. Therefore, the operations for image compression including the encoding are performed on the inputted image data even in the case where the compression ratio of the inputted image data is allowed to be further increased, so that calculation burden of the image compression processing is high.